218 research outputs found
Shape transition and oblate-prolate coexistence in N=Z fpg-shell nuclei
Nuclear shape transition and oblate-prolate coexistence in nuclei are
investigated within the configuration space (, ,
, and ). We perform shell model calculations for Zn,
Ge, and Se and constrained Hartree-Fock (CHF) calculations for
Zn, Ge, Se, and Kr, employing an effective pairing
plus quadrupole residual interaction with monopole interactions. The shell
model calculations reproduce well the experimental energy levels of these
nuclei. From the analysis of potential energy surface in the CHF calculations,
we found shape transition from prolate to oblate deformation in these
nuclei and oblate-prolate coexistence at Se. The ground state of
Se has oblate shape, while the shape of Zn and Ge are
prolate. It is shown that the isovector matrix elements between and
orbits cause the oblate deformation for Se, and four-particle
four-hole () excitations are important for the oblate configuration.Comment: 6 pages, 5 figures, accepted for publication in Phys. Rev.
Extrapolation method in shell model calculations with deformed basis
An extrapolation method in shell model calculations with deformed basis is
presented, which uses a scaling property of energy and energy variance for a
series of systematically approximated wave functions to the true one. Such
approximated wave functions are given by variation-after-projection method
concerning the full angular momentum projection. This extrapolation needs
energy variance, which amounts to the calculation of expectation value of
square of Hamiltonian . We present the method to evaluate this
matrix element and show that large reduction of its numerical computation can
be done by taking an advantage of time-reversal symmetry. The numerical tests
are presented for shell calculations with a realistic residual
interaction.Comment: 5 pages, 2 figures, accepted for publication in Phys. Rev.
Nuclear Shell Model by the Quantum Monte Carlo Diagonalization Method
The feasibility of shell-model calculations is radically extended by the
Quantum Monte Carlo Diagonalization method with various essential improvements.
The major improvements are made in the sampling for the generation of
shell-model basis vectors, and in the restoration of symmetries such as angular
momentum and isospin. Consequently the level structure of low-lying states can
be studied with realistic interactions. After testing this method on Mg,
we present first results for energy levels and properties of Ge,
indicating its large and -soft deformation.Comment: 12 pages, RevTex, 2 figures, to be published in Physical Review
Letter
Experimental evidence for 56Ni-core breaking from the low-spin structure of the N=Z nucleus 58Cu
Low-spin states in the odd-odd N=Z nucleus 58Cu were investigated with the
58Ni(p,n gamma)58Cu fusion evaporation reaction at the FN-tandem accelerator in
Cologne. Seventeen low spin states below 3.6 MeV and 17 new transitions were
observed. Ten multipole mixing ratios and 17 gamma-branching ratios were
determined for the first time. New detailed spectroscopic information on the
2+,2 state, the Isobaric Analogue State (IAS) of the 2+,1,T=1 state of 58Ni,
makes 58Cu the heaviest odd-odd N=Z nucleus with known B(E2;2+,T=1 --> 0+,T=1)
value. The 4^+ state at 2.751 MeV, observed here for the first time, is
identified as the IAS of the 4+,1,T=1 state in 58Ni. The new data are compared
to full pf-shell model calculations with the novel GXPF1 residual interaction
and to calculations within a pf5/2 configurational space with a residual
surface delta interaction. The role of the 56Ni core excitations for the
low-spin structure in 58Cu is discussed.Comment: 15 pages, 7 figures, submitted to Phys. Rev.
Effective mass staircase and the Fermi liquid parameters for the fractional quantum Hall composite fermions
Effective mass of the composite fermion in the fractional quantum Hall
system, which is of purely interaction originated, is shown, from a numerical
study, to exhibit a curious nonmonotonic behavior with a staircase correlated
with the number (=2,4,...) of attached flux quanta. This is surprising since
the usual composite-fermion picture predicts a smooth behavior. On top of that,
significant interactions are shown to exist between composite fermions, where
the excitation spectrum is accurately reproduced in terms of Landau's Fermi
liquid picture with negative (i.e., Hund's type) orbital and spin exchange
interactions.Comment: 4 pages, 3 figures, REVTe
Variational Monte Carlo Study of Electron Differentiation around Mott Transition
We study ground-state properties of the two-dimensional Hubbard model at half
filling by improving variational Monte Carlo method and by implementing
quantum-number projection and multi-variable optimization. The improved
variational wave function enables a highly accurate description of the Mott
transition and strong fluctuations in metals. We clarify how anomalous metals
appear near the first-order Mott transition. The double occupancy stays nearly
constant as a function of the on-site Coulomb interaction in the metallic phase
near the Mott transition in agreement with the previous unbiased results. This
unconventional metal at half filling is stabilized by a formation of
``electron-like pockets'' coexisting with an arc structure, which leads to a
prominent differentiation of electrons in momentum space. An abrupt collapse of
the ``pocket'' and ``arc'' drives the first-order Mott transition.Comment: 4 pages, 3 figure
Critical Casimir effect and wetting by helium mixtures
We have measured the contact angle of the interface of phase-separated
He-He mixtures against a sapphire window. We have found that this
angle is finite and does not tend to zero when the temperature approaches
, the temperature of the tri-critical point. On the contrary, it increases
with temperature. This behavior is a remarkable exception to what is generally
observed near critical points, i.e. "critical point wetting''. We propose that
it is a consequence of the "critical Casimir effect'' which leads to an
effective attraction of the He-He interface by the sapphire near
.Comment: submitted july 13 (2002), published march 20 (2003
Magic numbers in exotic nuclei and spin-isospin properties of {\it NN} interaction
The magic numbers in exotic nuclei are discussed, and their novel origin is
shown to be the spin-isospin dependent part of the nucleon-nucleon interaction
in nuclei. The importance and robustness of this mechanism is shown in terms of
meson exchange, G-matrix and QCD theories. In neutron-rich exotic nuclei, magic
numbers such as N = 8, 20, etc. can disappear, while N = 6, 16, etc. arise,
affecting the structure of lightest exotic nuclei to nucleosynthesis of heavy
elements.Comment: 4 pages, 3 figures, revte
Level spacing distribution of pseudointegrable billiard
In this paper, we examine the level spacing distribution of the
rectangular billiard with a single point-like scatterer, which is known as
pseudointegrable. It is shown that the observed is a new type, which is
quite different from the previous conclusion. Even in the strong coupling
limit, the Poisson-like behavior rather than Wigner-like is seen for ,
although the level repulsion still remains in the small region. The
difference from the previous works is analyzed in detail.Comment: 11 pages, REVTeX file, 3 PostScript Figure
Mott Transition in the Hubbard Model on Checkerboard Lattice
We investigate the bandwidth-controlled Mott transition in the Hubbard model
on the checkerboard lattice at half filling using the path-integral
renormalization group (PIRG) method. It is demonstrated that the system
undergoes a first-order phase transition to the plaquette-singlet insulating
phase at a finite Hubbard interaction. This conclusion is drawn via a detailed
analysis of the spin and charge correlations around the phase transition point
by means of the PIRG method aided with a new iteration scheme introduced in
this paper.Comment: 7pages, 9figures, accepted for publication in J. Phys. Soc. Jp
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